During the past several years, a miniature (0.5 mm. diameter) fiber optic pH probe has been developed for physiological use. The first generation of probes and support equipment demonstrated the feasibility of optically measuring pH with a pH sensitive dye, and made many subleties associated with the probe construction and use apparent. The next generation of probes improved upon the original design to reduce their response time to a step change in pH to less than 60 seconds, to extend the range of the probe to between pH 6.5 and 8, and to provide for easy and atraumatic insertion of probes into the beating canine myocardium. Furthermore, the support system was changed to consist of a digital computer and associated hardware capable of collecting data from 5 probes simultaneously. Within the past year, a series of experiments using the improved fiber optic pH measurement system to measure pH in the contracting canine myocardium has provided cardiologists with in vivo, real-time measurements of intramyocardial pH. Such inforamtion is of use to experimenters interested in evaluating drugs which affect tissue perfusion, obstetricians interested in monitoring fetal scalp pH and biomedical researchers in general. Recently, a new suppport system has been constructed with uses green/red LED's to replace the larger and more costly quartz halogen light source and motor-driven filter wheel of the second generation device. The new system utilizes a microprocessor to collect, analyze and display the data. Commercially available fiber optic connectors are now used and improve calibration procedures have been implemented.